The dominant is the fifth note on a seven note scale (a heptatonic scale).
The dominant of the C major scale, for example, is G.
The note is called dominant as in most common heptatonic scales it is the most pronounced harmonic of the tonic (the first note and root) other than the tonic itself. In the example above, the frequency of G one octave up is equal or approximately equal, depending on the tuning, to the frequency of C multiplied by 3. Thus, G is the closest odd order harmonic of C (the closest even order harmonic is just C one octave up).